How does that work for clients without the concept of a tab
Discussion
By
TAB I mean a simple box to check next to the post button. That's all.
OK that sounds more like a checkbox or other slider ui element to set content-warning tag specified by NIP-36. For example, amethyst has this

But not all clients support all NIPS and textually tagging something as NSFW has traditionally been an indicator, and some clients do filter on that without the content-warning tag being set.