This character is a Other Symbol and is commonly used, that is, in no specific script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Other Neutral. When changing direction it is not mirrored. The word that U+2621 forms with similar adjacent characters prevents a line break inside it.
The Wikipedia has the following information about this codepoint:
The dangerous bend or caution symbol ☡ (U+2621☡CAUTION SIGN) was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.
Representations
System
Representation (click value to copy)
Nº
9761
UTF-8
E2 98 A1
UTF-16
26 21
UTF-32
00 00 26 21
URL-Quoted
%E2%98%A1
HTML hex reference
☡
Wrong windows-1252 Mojibake
☡
Encoding: GB18030 (hex bytes)
81 37 A5 37
RFC 5137
\u'2621'
Bash and Zsh inside echo -e
\u2621
C and C++
\u2621
C#
\u2621
CSS
\002621
Excel
=UNICHAR(9761)
Go
\u2621
JavaScript
\u2621
Modern JavaScript since ES6
\u{2621}
JSON
\u2621
Java
\u2621
Lua
\u{2621}
Matlab
char(9761)
Perl
"\x{2621}"
PHP
\u{2621}
PostgreSQL
U&'\2621'
PowerShell
`u{2621}
Python
\u2621
Ruby
\u{2621}
Rust
\u{2621}
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