The 2.x.x releases will support PHP >= 8.1 which is typical for most source code & docker image installs now.
The 1.x.x releases will be the only ones supporting PHP >= 7.4 in the future.
The main difference compared to release 1.5.4 is switching from the (very) old TbsZip class to the modern maennchen/zipstream-php package to update EPub files on the fly, via the dependency on package mikespub/php-epub-meta release 2.x instead of 1.x
Also included are a few more improvements on the bootstrap5 template - see pull request #35 from @dunxd
Full Changelog: 1.5.4...2.0.1