Will Mordekaiser retain his anti-assassin playstyle when he gets updated?
Probably.
If he gets updated/reworked, he'll probably become more of a tanky DPS champion and less of a glass-cannon-that-becomes-tanky-when-he-bursts deal. Currently, he's pretty similar to pre-rework
in that his aesthetic is not in line with his build path/play style. More likely than not Riot is going to make him feel more like the Sion of now.
Which is good news if you want him to counter assassins, because that's by and large what tanky DPS champions do.
Assassins are all about speed. They use speed to survive, and deliver their burst of damage fast (so using speed to survive works). Speed is great vs. range as a defensive mechanic, and both tend to cancel one-another out in a 1v1.
Speed is not great vs. raw defense, as moving around doesn't give much of an advantage: especially for melee-range assassins vs. melee-range fighters. Fighters want you in melee range.
Burst also isn't that amazing vs. defense, as with burst and how it scales you generally want to take it to the point where you can do 110% of a squishie's HP and drop them in 1 combo. Vs. a fighter, the struggle is pretty real to accomplish that. And Mordekaiser will likely retain being very sustained-damage heavy in any foreseeable rework, which is just going to be bad news bears for any assassin he runs into.