There's an actual program, and a saas version. The saas version isn't worth paying for (imo) over using AnkiWeb unless you wanted preformatted decks, and the most up to date version of the algorithm.
I've experimented with using the website (mostly because I'm a fan of the software). It's mostly okay. I've been using Supermemo (the actual app) for a few years now on a daily basis. The app itself has a trial if you can find a download link somewhere, and I believe the site might be free if you import your own decks.
As far as lock in, I believe you are also locked in unfortunately. This would be another reason to go with Anki or another solution if you just want flashcards.
We are talking about https://super-memo.com/. The supermemo.com/SuperMemo World is different. I never used the latter. I think they are managed by two different groups of people.
SuperMemo 15 is free. The latest version is paid software unfortunately.
And is there an export function to avoid vendor lock-in? (Don't want to tempt my luck and waste my time scripting or writing a GDPR request)
and are we talking about https://super-memo.com/ or https://www.supermemo.com/ ?