Both examples are guessing and should not have been done
Though both had legitimate reasons for being exceptions to the general rule it sets a precedent that we probably don't want and gains us very little. In both cases waiting an additional 10 minutes would have been enough for OP to clarify.
The UA Alchemist Question
As I commented after my rollback, UA could mean something completely different in a different system. Alchemist and 2019 are only useful to confirm a belief not determine the system to begin with. Between the three you could be 95-98% confident but that still leaves room for error.
The 5.0 Hexblade Question
DND 5e is not the only 5th edition of an rpg. Guessing a system based on a 5.0 in the question title is not a good idea. Though likely to be right based on hexblade and additional information this isn't something we should do.
As I said above, not guessing only costs us time. We can wait for the OP to respond and then give quality answers. If they never reply then there's nothing lost, since they wouldn't have seen the answers anyway and it's better for the question to be closed and eventually deleted as abandoned.
If we guess the system other users may weigh in with answers based on that guess. In the 2% of cases that you made a mistake these answerers have wasted their time. You have also wasted the time of the mods who have to clean up the post once the OP does return and edit in the correct system.
It is always better to close a question and wait for clarification then to edit or answer an unclear question.