Ref T13588. See PHI2228. Under PHP8.2 and newer, calls to "utf8_decode()" raise a deprecation warning.
The behavior of this function probably isn't great under any PHP version since it UTF8 space into ISO-8859-1 space, which isn't an operation you can really implement "correctly" in the general case. For the specific narrow case of counting characters, as here, you could probably do worse, but I think the PHP upstream's deprecation of this function is entirely reasonable.
The fallback implementation for strlen, "phutil_utf8v()", should produce the correct result in all cases. The downside is that it's runtime UTF8 parsing, which may be slow. This function is called rarely and I don't think this will present a problem, but if it does I'd rather address it once it arises.