Since it seems likely that Drupal Classic isn't going to be an "official" project from DA, and since Backdrop is the most developed alternative we're going to have in 2025, I wonder if the maintainers of Backdrop could be "convinced" (ie, paid) to work on some kind of semi-drop-in API compatibility module such that D7 contrib modules are easier to port over. I tried and failed to do something like this when D8 first came out.
Caveat: I have not used Backdrop yet, so I don't know what changes to the API/codebase they've made. My example below is only an example! I don't know what all in backdrop is incompatible with current D7.
Here's how it could work:
Let's say that Backdrop changed the way node_load() worked. Maybe they added new arguments or some such. My proposal is there's a Backdrop module called "d7b", and all you need to do is add "d7b_" to the incompatible function names for it to work.
So, to port over contrib modules that call node_load(), you'd simply find & replace it with d7b_node_load(). Similarly you could do that with any core API functions that are no longer compatible. Theoretically, the Backdrop "d7b" module would eventually contains hundreds of these helper functions that replicate how the original API worked for D7.
This would make it much easier to take existing modules for D7, run them through a quick script to convert the needed function names to the d7b_ equivalents, and presto, the contrib module now works in Backdrop.
I know this isn't going to work for everything, and I may be greatly over-simplifying things. Just wanted to toss in my 2 cents. Speaking of, I would gladly pay a "bounty" for any Backdrop development on such an effort. My business relies heavily on D7, and I will be forced to switch to Backdrop when D7 reaches EOL. I'd love for Backdrop to be a relatively smooth transition.
I'm not in any way associated with this company, but I found this:
https://www.herodevs.com/support/nes-drupal
Claims to support Drupal 7 with "never-ending support". Prices not listed at the time of this post.
I run a software company, and use D7 extensively. I've been programming modules for D7 since it went live, and I basically program some kind of helper module for every site I create. Some sites are simple; some are VERY complex (ex: CRM's, medical EHR's, etc) with 10's of thousands of lines of code that will only work in D7. I have indeed tried module development in D9 and it was a nightmare compared to D7. So many little yaml files, so much rearranged and convoluted. Entire classes for every form?! Is it possible? Sure. But who has the time or money to spend recreating a site/product that already exists and works? Why take the risk of introducing years of bugs to your customer base which would never notice the difference otherwise?
I, for one, will probably never migrate to D9/10/11/whatever. Navigating symphony and composer is a huge pain compared to just unzipping files or editing simple .info files, implementing logical hook_ functions, etc. D7 was the peak for Drupal, and I'm afraid it's downhill from there.
My plan (if they actually do EOL it Jan 2025), is to first try to migrate all my custom code to Backdrop. If that fails, I'll pay for the service linked above or some other extended support. I'm hoping that as the real EOL gets closer and closer, enough people come together to keep the D7 (aka Drupal Classic) community alive.