if scratch accounts are inactive for long enough will they be deleted by st?

No. In fact, that idea has actually been rejected by the Scratch Team.

From The Official List of Rejected Suggestions:

Za-Chary wrote:

8.5 Recycle usernames or delete inactive accounts
The only potential benefit to these suggestions is that Scratchers could reuse usernames. However, this is problematic in several ways. Deleting inactive accounts might mean that the user's projects get deleted, which would be extremely unfortunate should they decide to return to Scratch. Even if the account is already deleted, someone may attempt to impersonate the deleted user with the new username. In addition, there are reasons that the Scratch Team may need to refer back to the deleted account.

However, deleting accounts with absolutely no activity since account creation is NOT rejected. (Note, however, that some accounts may have had activity even if it doesn't look like they do.) You can discuss it on this topic.

AIWizzard wrote:

No, Scratch accounts are not deleted for inactivity. The Scratch Team (ST) generally does not delete accounts simply because they have been inactive for a long time. Accounts remain accessible even if they haven't been used for years.

However:

If a user requests their account to be deleted, the Scratch Team can close it.
Accounts violating the Community Guidelines may be permanently banned.
Inactive accounts are usually safe, so you should be able to log back in whenever you want! Let me know if you need help recovering an old account.

Accounts violating the community guidelines are NOT always permanently banned. It depends on the severity. Sometimes it’s a temp ban but sometimes it’s a perma ban

Ok thanks I don’t need any help with old acount but thanks for info too both of you I’m kinda new to scratch discussions not to scratch been around a year just on this acount but to scratch discussion’s yes