Couple of issues that need to be resolved for a D11 compatible release
web/modules/contrib/search_api_synonym/search_api_synonym.info.yml 11 Value of core_version_requirement: ^8 || ^9 || ^10 is not compatible with the next major version of Drupal core. See https://drupal.org/node/3070687.
web/modules/contrib/search_api_synonym/src/Export/ExportPluginManager.php 231 Fetching deprecated class constant EXISTS_REPLACE of interface Drupal\Core\File\FileSystemInterface. Deprecated in drupal:10.3.0 and is removed from drupal:12.0.0. Use Drupal\Core\File\FileExists::Replace instead.
web/modules/contrib/search_api_synonym/src/Form/SynonymImportForm.php 191 Fetching deprecated class constant EXISTS_RENAME of interface Drupal\Core\File\FileSystemInterface. Deprecated in drupal:10.3.0 and is removed from drupal:12.0.0. Use Drupal\Core\File\FileExists::Rename instead.
Active
2.0
Code
We are going to need to branch this away from Drupal 8, because FileExists doesn't exist there.
Id be inclined to create a 3.x branch that is D10+ and 2.x be considered legacy.
Excellent, we was looking for a branch that supported and was a little confused why 📌 Automated Drupal 11 compatibility fixes for search_api_synonym Active was closed.