My Discord <-> IRC <-> Slack <-> Matrix bridge with Matterbridge [0] still works as of today, I didn't notice anything being shut down.

[0]: https://github.com/42wim/matterbridge

Slack’s own IRC bridge was quite terrible and had been for years. It had multiple unimplemented features and multiple bad bugs. Evidently they decided they had done enough embracing and extending, and so rather than fixing it it was time to extinguish.