PHP7.4 is deprecated - that doesn't mean every site and server are running at that version. How many sites are still running under PHP5.6 or less? Even getting to that version could be an adventure. I've found a number of ImpressCMS sites that will have to take extra steps before they upgrade, some of mine are included in this list.

At a minimum, they will have to upload the new version and switch their server to the new version of PHP in order to complete the upgrade. If they haven't switched to PDO, they will have to do that, too, before they proceed. I use Formulize which implemented PDO independent of the core and activating PDO for the site causes problems with the module.

These changes are to be expected - I feel we need to be very open about what needs to happen and help them anticipate and navigate the requirements.

I've been thinking about a pre-upgrade script that could be uploaded to their site, or added as a custom block, that will do check the preliminaries a little more closely. We can't really do this in the current system block because they would have to upgrade to get the added tests. It could also scan for the use of deprecated functions and methods that were removed in 1.4 and the upcoming 1.5. The deprecated messages showing when debug is turned on is also a good place to make sure you're ready to move ahead.