I'm running Big Sur 11.6 and I've just had a notification stating that I've not performed any TM backup for XX days. It seems Apple has reintroduced these notifications as it's been years since I saw them last.
If it went away at some point, I don't think it came back recently. I've gotten that message for the last few years once I started swapping external backup drives.
I've been getting these notifications occasionally ever since I've been using Time Machine with Mojave...so I don't think they ever went away. I use a NAS for my Time Machine backup, and I'll get the notification after I've been away for about 10 days.
Perhaps it has to do with how big your internal drive is. Did you get a new computer with a larger drive in those years since you last saw the notification? The larger the drive, the more local snapshots it can hold before it needs to offload them into Time Machine (and show the notification).
It's kinda weird, I'm almost certain I came across some threads mentioning that they had disappeared. Myself included, I stopped receiving them. I don't know, I may be hallucinating.