As per What to do when an edit guesses the system being used rather than waiting for the querent to clarify?, editors should not guess the missing system tag (e.g. dnd-3.5e) without asking the question maker, in case they are wrong. This is reasonable, as it's hard to be 100% sure (e.g. Pathfinder is easily mistaken for D&D 3.5).
However, in this question, Can you use darts as improvised "light melee weapons" to trigger Two-Weapon Fighting?, the system is determinable with 100% certainty. The user quotes the Two-Weapon Fighting rules on page 195 of the Player's Handbook, which are identical verbatim to the Two-Weapon Fighting rules on page 195 of the D&D 5th edition Player's Handbook. In this case, it's 100% certain that the player is asking about D&D 5th edition.
In those cases where the system is 100% certain, is it reasonable for an editor to add the correct system tag?