According to my research, it was supposed to be supported starting from version 24.
I tested your code on the most recent Firefox browser and it worked without any problems.
While the documentation mentions that it MUST support GMT and MAY support IANA time zones, there is no specific mention of offsets:
The only value implementations are required to recognize is "UTC"; otherwise, they default to the system's time zone. Implementations may also accept the names of time zones from the IANA database, such as "Asia/Shanghai", "Asia/Kolkata", "America/New_York".
In theory, implementations could choose not to support any time zones at all, but in practice, most applications should support IANA time zones.
You can attempt specifying the IANA timezone offset name:
new Intl.DateTimeFormat('en', {
dateStyle: 'long',
timeStyle: 'long',
timeZone: 'Etc/GMT-5',
}).format(new Date());
"February 18, 2024 at 3:42:40 AM GMT+5"
If that doesn't work, there might be some issue with your configuration. Keep in mind that I used the inverse offset (-5 instead of +5). This is how the IANA timezone Etc offsets are named: they represent the opposite of the actual offset (+0500 for Etc/GMT-5 and -0500 for Etc/GMT+5).