Inside of strings or comments or as an encoding is close to universal now, but for wide support for operators & variable names I would generally it isn’t. Some languages straight up do not support non ASCII like OCaml, others only support bicameral scripts like PureScript, but others like JavaScript can support Unicode for variable names but doesn’t support defining infix operators or uses Unicode for any existing operators. Raku is probably the most Unicode-friendly language, & some of the mathier ones like Agda as well.