Every archiving tool out there makes trade-offs about what is archived and how. No one preserves the raw TLS encrypted H3 traffic because that's not useful. When you browse through an archiving MITM proxy, there are different trade-offs: there's an extra HTTP connection involved (that's not stored), a fake MITM cert, and a downgrade of H2/H3 connection to HTTP/1 (some sites serve different content via H2 vs HTTP/1.1, can detect differences, etc...)
The web is best-effort, and so is archiving the web.
Unfortunately, there is not much we can do about transfer-encoding, but the data is otherwise exactly as is returned from the browser. Browsertrix uses the browser to create web archives, so users get an accurate representation of what they see in their browser, which is generally what people want from archives.
We do the best we can with a limited standard that is difficult to modify. Archiving is always lossy, we try to reduce that as much as possible, but there are limits. People create web archives because they care about not losing their stuff online, not because they need an accurate record of transfer-encoding property in an HTTP connection. If storing the transfer-encoding is the most important thing, then yes, there are better tools for that.
This isn't really true, our tools do not just modify response data for no reason!
Our tools do the best that we can with an old format that is in use by many institutions. The WARC format does not account for H2/H3 data, which is used by most sites nowadays.
The goal of our (Webreocrder) tools is to preserve interactive web content with as much fidelity as possible and make them accessible/viewable in the browser. That means stripping TLS, H2/H3, sometimes forcing a certain video resolution, etc.. while preserving the authenticity and interactivity of the site. It can be a tricky balance.
If the goal is to preserve 'raw bytes sent over the network' you can use Wireshark / packet capture, but your archive won't necessarily be useful to a human.
The web is best-effort, and so is archiving the web.