[lsaferite]

Having dealt with utf8mb4 data being inserted into the utf8mb3 columns many many times in the past, I've never had a table "instantly go down". You either get silent truncation or a refusal to insert the data.

[eurasiantiger]

Well, your applications haven’t used a serialized or JSON column. That’s how you go from truncation to downtime.

That said, I do remember this being an issue even with plain text.