Whether or not it is planned obsolescence is not a technical question but one of intention.
If shorter support periods were motivated by an expectation that customers would feel forced to buy new hardware rather than launching into a complex Linux migration, then that would be planned obsolescence.
But I think it's not that simple. Limited support periods can always be justified by pointing to difficulties further upstream along the supply chain and to the fact that longer support periods cause degraded security and higher cost.
You could still argue that putting more regulatory pressure on hardware makers and platforms would indirectly lead to longer support periods throughout the industry.
On the other hand, you'd have to ask why consumers are not creating this pressure themselves. Has the market converged to a steady state where consumers no longer have a choice? If that was the case I would be all for regulation to create choice.
We should be under no illusion though that longer support periods come at a cost. If longer support is mandated by regulators then everyone will have to pay more. If we leave it to consumers then only those who value longer support will pay for it.
Buying the cheapest hardware possible and then petitioning vendors to fund longer support periods does not seem like a solution to me.
We have to make up our minds on whether using hardware for longer is for the common good and we all carry the costs, or if it is a feature of some products but not others and those who value it have to pay up.