About 10 years of photos have disappeared from my photo library.

What has changed, directly before the photos vanished from your Photos Library?

Occasionally newer photos seem to have vanished, when Photos has opened a different, older Photos Library, instead of your main library.

You can check, which Photos Library is currently open, by looking at the Photos > Settings > General tab. There you can reveal your Photos Library in the Finder. Is the correct library selected?

Photos is sometimes grouping years together in the Years view. Have you looked into the 2009 Year, if there are also photos from later Years?

Do you still have the old Mac or have you traded it in?

Do you still have backup drives from your old Mac, that you have used with the old Mac?