I’ve already encountered this once; I have a few best buy “smart light switches” that are HomeKit compatible; they continue to work in HomeKit because Apple (peace be upon Jobs) demanded that HomeKit work without cloud access.
I have not tried to see if they can be JOINED to HomeKit now that the Best Buy servers are gone (and to Best Buy’s credit, they gave every customer a gift card equal to the FULL PURCHASE PRICE of the hot garbage (and yes, spellchecker, hot is fine for IoT here)) but at least they haven’t stopped working (and when they do, they’ll still work as an physical switch).
and once you get done with the “oooo I can tell Siri to turn off the lights” it’s just not really something used much in “real day-to-day life”.