“Apple deliberately slows old machines to force upgrades”

The origin is genuine. Apple did throttle peak performance on iPhones with degraded batteries to prevent unexpected shutdowns, failed to explain it, was fined over it, and later added a user-facing toggle.

That was battery management on phones. It is not evidence of a policy of slowing Macs, and no comparable finding exists for them.

What does make an older Mac feel slow is ordinary and true of every computer: newer software expects more memory, and a machine whose memory you cannot upgrade ages faster than one you can.