These are real Unicode characters, not fonts
The styled text these tools produce is not formatting — it is substitution with different Unicode code points. The mathematical alphanumeric symbols block contains complete alphabets in bold, italic, script, fraktur, double-struck and monospace forms, and a "bold" social media bio is made of those characters rather than a bold font.
They exist for mathematics, where the distinction between a variable written in italic and one in double-struck is semantic — ℝ means the real numbers and R does not. Unicode encoded them because the difference carries meaning that formatting cannot express in plain text.
This is why the styling survives being pasted into places that strip formatting, which is the whole appeal. It is also why the characters cannot be searched, sorted or spell-checked as ordinary letters: to software they are unrelated symbols that happen to look like letters.
The accessibility cost is severe
A screen reader encountering these characters does not see styled letters. Depending on the reader and its character tables, it either reads each character's Unicode name — announcing "mathematical bold capital H, mathematical bold small e" and so on — or skips them entirely, leaving silence where the text should be.
For a social media display name, this means a blind user hears a string of symbol names instead of a name. For a bio or a post, it means the content is unavailable. This is not a minor degradation; it is a complete failure to convey the text, and it affects the same people for whom the visual styling has no benefit at all.
Search is affected equally. A profile with a stylised name will not be found by searching the ordinary spelling, because the characters do not match. The same applies to browser find-in-page, autocomplete, and any system that indexes text.
Using it without causing problems
The reasonable use is short and decorative where the same information exists in plain text elsewhere — a single emphasised word in a post whose meaning does not depend on it, or ornamentation around text that is itself readable. The unreasonable use is a display name, a heading, a call to action, or anything a reader must be able to parse.
Never use these characters where they carry essential information, and never use them in a document, email or web page, where actual formatting is available and works properly for everyone. Real bold in HTML or a word processor conveys emphasis to a screen reader; substituted characters do not.
Be aware that platforms are increasingly filtering them. Several social networks now normalise or reject stylised characters in display names specifically because they are used to evade moderation and impersonate accounts, and some search systems strip them entirely. A name that renders decoratively today may be normalised tomorrow.