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WordPress version 5.5 is the earliest version, I will also not make a version compatibale with anything earlier than that due to security issues that were present in earlier versions of WordPress.
If you want to experiment you can change the Requires at least: 5.5 in the readme.txt to Requires at least: 4.9, but I will not support versions lower than 5.5, you will also likely not be able to update it when I release updates.
I understand ClassicPress is different and may contain security & performanc updates, however, this is an official WordPress plugin, thus I will not support it on anything other than WordPress, and nothing earlier than version 5.5.