On a Ubuntu host, Classic zabbix agent listens for default port 10050, Agent2 listens for 10060.
Both agents are fresh compiled v6.0.15rc1

# zabbix_get -s localhost -p 10050 -k 'web.page.get[http://localhost/dev/raw]'
HTTP/1.1 200 OK
Date: Thu, 16 Mar 2023 10:30:45 GMT
Server: Apache/2.4.52 (Ubuntu)
Last-Modified: Thu, 16 Mar 2023 09:56:38 GMT
ETag: "d-5f7017a0cdc99"
Accept-Ranges: bytes
Content-Length: 13

....

# zabbix_get -s localhost -p 10060 -k 'web.page.get[http://localhost/dev/raw]'
HTTP/1.1 200 OK
Connection: close
Content-Length: 13
Accept-Ranges: bytes
Date: Thu, 16 Mar 2023 10:30:48 GMT
Etag: "d-5f7017a0cdc99"
Last-Modified: Thu, 16 Mar 2023 09:56:38 GMT
Server: Apache/2.4.52 (Ubuntu)

...

1. Extra request header:

Classic does not specify it.

Agent2:

Connection: close

This causes the header returned too, which makes a difference. May break preprocessing steps.

2. Extra request header:
Classic:

Accept: */*

Agent2 does not specify it.
Assumption - potentially it may cause a differenrce in data.

3. Different request header:
Classic:

Accept-Encoding: deflate, gzip, br, zstd

Agent2:

Accept-Encoding: gzip

All these differences may cause troubles when zabbix user migrate from Classic agent to Agent2.

p.s. I really have an issue on an installation, where returned data is different for the agents, but hat investigation is not complete yet.

Note - recently there was a related case about "Accept-Encoding" header, keep it in mind! - ZBX-21765