Though I think American English tends to pronounce 'ton' as 'tun', would that be right, or are there significant regional accent variations?
Anyway, refer sibling comments on common usage and cultural defaults - and I'll re-emphasize that it works really well for written communication. Like this.
It's extremely rare (don't think I've ever heard it) - except maybe in some industries still, I don't know - but there's an interesting history of advocacy/use of saying 'ton-E' or 'tunnie', to disambiguate the same way AmE pronounces it 'metric ton'.
I've never heard it pronounced as a two-syllable word in the UK, but I confess to only very occasionally travelling north of Luton.
I don't think it was ever common parlance, just important to disambiguate in some industries. Similarly I think 'thou' (-sandth of an inch) is preferred over 'mil' since that's such a common abbreviation of 'millimetre' (which =39thou, so you want to get that right).