https://www.gamespot.com/articles/pokemon-go-last-chance-to-get-legendary-registeel/1100-6461120/

If you still haven't captured Registeel in Pokemon Go, you only have a few more days to do so. The Legendary Steel-type Pokemon is scheduled to leave Raid Battles later this week, on August 16, making this your last opportunity to add one to your collection for the foreseeable future.

Like Regice (which appeared in Raid Battles earlier this summer), Registeel is one of the three Legendary titans introduced in Pokemon Ruby and Sapphire for Game Boy Advance. To catch it in Pokemon Go, you'll first need to team up with other players at a Gym where a Registeel Raid Battle is taking place and challenge the Legendary; if you manage to defeat it, then you'll be rewarded with a chance to capture it.