I like to call it "premature obsolescence". I don't believe that the manufacturers actively try to make your old phone get slower.
Rather the opposite: they don't try to keep your old phone working. The devs probably have the latest phone, and write code that works well with them. This code can be inefficient, as long as it works well on the newer phone, even if it makes older phones slower.
Maybe I am being pedantic, but I think it is different: it is _not_ that manufacturers actively pay their devs to make the phones slower (which would be planned obsolescence), but rather they _do not pay_ the devs to make sure the old phones _do not get slower_. The latter is "passive", that's (to me) premature obsolescence: the manufacturer could fight premature obsolescence, but it has a cost (and obviously they don't want to pay for it).