A School of Necromancy Wizard at 6th level gains the Undead Thralls ability, which reads:
At 6th level, you add the animate dead spell to your spellbook if it is not there already. When you cast animate dead, you can target one additional corpse or pile of bones, creating another zombie or skeleton, as appropriate.
Whenever you create an undead using a necromancy spell, it has additional benefits:
- The creature’s hit point maximum is increased by an amount equal to your wizard level.
- The creature adds your proficiency bonus to its weapon damage rolls.
This is naturally most obviously relevant to Animate Dead itself, which starts with:
This spell creates an undead servant.
So Animate Dead is extremely explicit in triggering the ability. However, Summon Undead doesn't have the word "create" anywhere in its description so it's less clear.
Looking further, I found several other necromancy spells which result in new undead, and none of them have "create" or similar in their descriptions either, and I would assume the class feature would simply say Animate Dead if that's the only spell that it works with, which would mean Summon Undead is probably fine too... But (at least on D&D Beyond) most of the other spells which make undead are tagged as "Creation" spells, which makes sense for them to be considered as "creating undead" for the class feature, whereas Summon Undead is tagged as Summoning instead, which also makes sense given the name. I'm pretty sure D&D Beyond tags aren't actually rules, but I can see the logic that "Summoning" spells are taking a creature that already exists on another plane and moving it to the material plane, whereas "Creation" spells do something different and thus qualify as "creating" something.
So, personally I would rule that Summon Undead triggers Undead Thrall, but I can see other interpretations and I wonder whether there's a more official way to determine the meaning of "create an undead".