Called 'The Mullet' no idea where that name came from but its been about for an age.You can sometimes get the best of both worlds by fitting a large front wheel and a smaller rear wheel. I always liked the more 'relaxed' or 'comfort' geometry that results from this.(Front higher, easier to put your feet on the ground, more stable etc). I am riding at the moment a Carrera Vengence with a 27.5 front and a rear 26 hub motor (XF15).
Originally it was 26/24, mainly on the early DH types, then when 27.5" came in people put the 27.5" on the front, with a 26" rear, but I suspect they only did that because they didnt want the more expensive outlay of a rear 27.5".
29" swept the field clear of offroad wannabees and 27.5" died a death.
26" became the standard size for teenage kids bikes, along with 24" for the smaller kid.
27.5" is still around, but im not sure why really. Maybe frame manufacturers are just lazy