Frankly, the supposed variability of the Accept header never really sat all that well with me; in practice I much prefer working with explicitly versioned endpoints — one of the most infuriating things is having to hardcode "Accept: text/x-myorgname-custom-json-blob-v4" because omitting it would produce "406 Not Acceptable". Bonus points if that's the only Accept header the service would ever accept in all of three years of it working before being decommissioned. Double bonus points if v5 would be introduced behind a separate URI anyway (and it, too, would require precisely "Accept: text/x-myorgname-custom-json-blob-v5" and nothing else).