It's impossible to delete messages/use Undiscord on archived threads. A remedy would be nice.

[ZaptorZap]

When Undiscord attempts to delete messages with this condition, Discord's API throws code 50083: "Thread is archived".

To automate deletions of these messages, provided you have the rights to open and close threads, Undiscord could temporarily reopen these threads to delete their contents. This could be disruptive to the active thread list, and server members might be confused by new threads that have died off months to a year ago being frantically opened and closed, so this support would be optional—disabled by default. If it is disabled, you shouldn't even attempt to delete messages under known archived thread channel IDs after the issue has been diagnosed, skipping all eligible messages which share that ID.

Consider specifically locked threads: Once unlocked you may have to lock them once more, instead of regularly archiving them. Since an operation may leave a previously locked thread open for interactions for long durations of time, you may also want an option that skips messages in locked threads entirely.

Relevant API documentation is here and here.