There is neither written guideline nor consensus but it has already been discussed.
Some (most of?) URL relationship types do have begin/end dates. See STYLE-580 for examples. I usually set begin/end dates accordingly. Other URL relationship types don’t, either at purpose or not.
This is more than a STYLE issue. These attributes can be edited from the URL entity page only, which you can access using the info links (in the relations tab and in the editing history). This is a pretty tedious task and should be editable from both sides as requested in MBS-3774.
From there, linking to the Wayback Machine should probably be automatically be done by the MusicBrainz server itself, not by editors.
Besides, there is a plan to save every link to the Wayback Machine as suggested in MBS-9009.