As I was looking for the solution (i thought I had it with last_updated) to raise a notification on my phone when any of sensors required for heating goes dead so I can somehow react (prevent some disaster) I stumbled on WTH discussion: WTH, how can I know when the last (unchanged) sensor value was received? - Month of “What the heck?!” - Home Assistant Community (home-assistant.io)
which turned out to be counterproductive. So I went online to find any workaround and found nothing of use (nothing I can set up fast when setting new sensors).
The only way would be ping <=> pong or somekind of heartbeat I can set up as scripts/automations/embed in sensor (ok for me to do it but not really user friendly) BUT that is relevant only to sensors/devices on ethernet (nothing for zigbee/esphome/ble/… devices). There was a suggestion: Use ESPHome, it has a expire_after configuration option.
But it works ONLY if you have mqtt component in configuration. Not sure why would I need mqtt if ESPhome has native API (which is faster, more reliable … )
I KNOW last_updated and last_changed CANNOT be reprogramed as ppl use them as they were designed from start. BUT we really need then the attrib something like: last_seen
Otherwise to all the devs of HA THANK YOU very much for all the good work you did/do/will 
