If datapacks add too many recipes to the game, no client can connect to it and they disconnect immediately. The reason is due to the RecipeBook packet being too big. It errors giving a DecoderException: Badly compressed packet - size of X is larger than protocol maximum of 2097152. 

I got a size of 8.3M by copy pasting the vanilla recipes into different namespaces. I had in total 1792 times the amount of recipes vanilla has. So the effective limit would be around ~450x the size of all vanilla recipes. 

Even if this is indeed a weird case, it is not impossible to attain this max in heavily modded or data-driven servers. Wanted to provide what I used to test, but it is too big, so I uploaded it to my drive, https://drive.google.com/file/d/1P9BJJSK5xmjCA2GQPXBCR6iNwjHdq0TW/view?usp=sharing. I calculate about 5 of these datapacks are needed to reproduce the bug (I had 16 when testing)

Also, since this is due to the recipe book, this probably does not effect only 1.15.