iOS 15.4 has been officially released since yesterday, along with other corresponding Apple updates for other devices, and brings with it all sorts of useful innovations and improvements.
However, if you use a smart home system from Bosch at home and have previously controlled your roller shutters using Siri commands, then the update to iOS 15 .4 will probably cause problems for you too.
At least some users are now reporting in the Bosch support forum that since the update to iOS 15.4 (probably also with HomePod OS 15.4) they can still control their shutters manually via HomeKit without any problems, but there is only a Siri voice command an error message.
Bosch seems informed and will probably already be working on an update, but the update to 15.4 is currently not recommended for all users without restrictions if voice control is mandatory.