Yes, running older core version will work for a while, but eventually will become too out of date to use. Hopefully core dont make it compulsory. Assuming that all knots nodes are eventualky forced to run a newer core node, I would think all of them will manually put op_return filter back on. Core will be be able to see this(when it happens). If they still make it complusory then they really are totally ignoring significant part of the community. 🤞