When saving posts, the first sentence is removed if it matches a regular expression. See this answer. The purpose is to automatically remove salutations from posts.
The word “hay” is a very uncommon English salutation, but it is a quite important word in Spanish, meaning “There is”. It is also a common word in English. It is included in the above-mentioned regular expression.
Should it be removed from it, and therefore allow questions and answers to start with “hay”?
Advantages: you could start a question in the Spanish Stack Exchange with “Hay una palabra que...” (“There is a word that...”), or an answer with “Hay dos respuestas para tu pregunta” (“Your question has two answers”). You could start a question on Puzzles SE with the words “Hay weighs 8 pounds per cubic foot”. Etcetera. Basically, you can start a post with a valid sentence starting with “hay”, of which there are many in Spanish.
Disadvantages: Somebody could start a question with the very uncommon salutation “hay!” and that salutation will have to be deleted.