Can confirm the issue. Sites on PHP 7.4.1 instantly fail with a 503.
This is the log from starting one of the affected sites:
{"thread":"main","class":"","message":"Compiling service configs...","level":"info","timestamp":"2020-11-15T10:13:47.038Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":"dyld: Symbol not found: _udat_patternCharToDateFormatField","timestamp":"2020-11-15T10:13:47.171Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Referenced from: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:47.172Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Expected in: %%userDataPath%%/lightning-services/php-7.4.1+10/bin/darwin/sbin/../dylibs/libicucore.A.dylib","timestamp":"2020-11-15T10:13:47.172Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" in /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:47.172Z"}
{"thread":"main","class":"Process","process":"nginx","pid":28774,"level":"info","message":"Killing process","timestamp":"2020-11-15T10:13:47.234Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":"dyld: Symbol not found: _udat_patternCharToDateFormatField","timestamp":"2020-11-15T10:13:47.700Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Referenced from: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:47.700Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Expected in: %%userDataPath%%/lightning-services/php-7.4.1+10/bin/darwin/sbin/../dylibs/libicucore.A.dylib","timestamp":"2020-11-15T10:13:47.701Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" in /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:47.701Z"}
{"thread":"main","class":"Process","process":"mysql","level":"warn","message":"2020-11-15T10:13:47.292746Z 0 [System] [MY-010116] [Server] %%resourcesPath%%/lightning-services/mysql-8.0.16+5/bin/darwin/bin/mysqld (mysqld 8.0.16) starting as process 30759","timestamp":"2020-11-15T10:13:48.112Z"}
{"thread":"main","class":"Process","process":"mysql","level":"warn","message":"2020-11-15T10:13:47.296679Z 0 [Warning] [MY-013243] [Server] --character-set-server: The character set UTF8MB3 is deprecated and will be removed in a future release. Please consider using UTF8MB4 instead.","timestamp":"2020-11-15T10:13:48.112Z"}
{"thread":"main","class":"Process","process":"mysql","level":"warn","message":"2020-11-15T10:13:47.297861Z 0 [Warning] [MY-010159] [Server] Setting lower_case_table_names=2 because file system for %%site.runData%%/mysql/data/ is case insensitive","timestamp":"2020-11-15T10:13:48.112Z"}
{"thread":"main","class":"Process","process":"mysql","level":"warn","message":"2020-11-15T10:13:48.081937Z 0 [Warning] [MY-010068] [Server] CA certificate ca.pem is self signed.","timestamp":"2020-11-15T10:13:48.113Z"}
{"thread":"main","class":"Process","process":"mysql","level":"warn","message":"2020-11-15T10:13:48.112020Z 0 [System] [MY-010931] [Server] %%resourcesPath%%/lightning-services/mysql-8.0.16+5/bin/darwin/bin/mysqld: ready for connections. Version: '8.0.16' socket: '%%site.runData%%/mysql/mysqld.sock' port: 10008 MySQL Community Server - GPL.","timestamp":"2020-11-15T10:13:48.113Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":"dyld: Symbol not found: _udat_patternCharToDateFormatField","timestamp":"2020-11-15T10:13:48.228Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Referenced from: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:48.229Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Expected in: %%userDataPath%%/lightning-services/php-7.4.1+10/bin/darwin/sbin/../dylibs/libicucore.A.dylib","timestamp":"2020-11-15T10:13:48.230Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" in /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:48.231Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":"dyld: Symbol not found: _udat_patternCharToDateFormatField","timestamp":"2020-11-15T10:13:48.751Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Referenced from: /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:48.751Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" Expected in: %%userDataPath%%/lightning-services/php-7.4.1+10/bin/darwin/sbin/../dylibs/libicucore.A.dylib","timestamp":"2020-11-15T10:13:48.751Z"}
{"thread":"main","class":"Process","process":"phpFpm","level":"warn","message":" in /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation","timestamp":"2020-11-15T10:13:48.752Z"}
Googling some of the errors shows that they are indeed connected to Big Sur and some have already been discussed in this forum, e.g. macOS 11 Big Sur Beta compatibility.