The bigger you are, the longer it takes people to notice, but I agree that 20 stone to 16 should be easily visible by now. I have lost three stone in a year (gone from just over 13 to almost 10, but was previously 14 stone at my biggest, 3 years ago. Very, very few people have commented.
I think there is are several reasons for this:
The people who think Wegovy is cheating. They don't want to congratulate you on a race that you won by unfair advantage.
The people who assume (probably correctly) that you are on Wegovy and they don't want to embarrass you by asking how you did it, in case you feel the need to lie.
They are still big and they are so envious they can't stand it. They see anyone achieving weight loss as a direct judgement on them.
They are big and are loud and proud about 'body confidence'. They see telling someone 'well done, you look great' is traitorous to those who claim to be proud and happy being obese, with no intention of changing.
There are those who think you should never comment on anyone's weight ever, regardless of whether it's because they've gained it or lost it. Either because of manners, or political correctness or not knowing the reason why someone may have lost weight, it may not be the welcome thing we all assume and they could be ill. I have some respect for this, as much as I love being told I look great. But I know I look great, and I know they know, so I don't really need to be told it. Knowing is enough.
We went for lunch with some friends we've not seen for a year recently. The wife said 'wow look at you! You look amazing! How much have to lost?' as soon as I took my coat off.
Her DH said nothing. She said 'Look! She's lost three stone!' He just grunted, pulled a face and looked uncomfortable as if she shouldn't be mentioning it.