Though they aren't time locked, there are a couple permanently missable events I was able to find on a wiki.
Clint's 6 heart event will not trigger if you've already seen Emily's 8 or 10 heart event, or are married to her.
Penny's 4 heart event will not trigger if you have bought the "Community
Upgrade" at Robin's after finishing the community center.
Buy Red Cabbage (RC), crocus, snow yam, fall crops, fall fish from traveling cart (TC). Get lucky with fruit. Have crop fairy speed up summer crops - Summer Y1 D2
Buy RC, crocus, snow yam, fall crops, fall fish, from TC. Get lucky with fruit - Summer Y1 D9
Buy RC, crocus, and snow yam from TC. Get lucky with fall crops - Fall Y1 D3