Drupal 9 and 10 compatible release?

Version 4 is quite a departure from previous versions, hence the major version.

4.0 supports D9, 4.1 supports D10 and later.

Since its the same major version, they are compatible with each other API/config wise, except for deprecations, etc.

Steps:

- While on D9, Update your constraints to support ^4. Get your D9 project working with 4.0 first.
- When you do your Drupal 10 update, simply add auditfiles to the update list, as in `composer update drupal/core drupal/auditfiles -W ...etc`. Depending on the command used you may just get the upgrade without explicitly specifying auditfiles.

Actually, I just realised the situation is even worse because 4.0 does not support PHP 8.

This is incorrect.

The project page also already summarises most of this.

Yes, the PHP version was a mistake, I corrected that.

OK. This is a shame. The project page doesn't document why it is not possible to have a release that is compatible with D9 and D10 at the same time. I already have three modules in this project that need to be updated at the same time as Drupal itself. The other two seem to have a hard technical restriction why it is impossible to have a release that is compatible to both D9 and D10.

Made a wrong edit concerning PHP versions, I failed to realise the quoted PHP versions are minimal versions.