I think the default filename should be {{PAGENAMEE}}.svg
Alternative:

• {{FULLPAGENAMEE}}.svg
• {{FULLPAGENAMEE}}-{{SITENAME}}.svg
• {{FULLPAGENAMEE}}-{{SITENAME}}-{{CONTENTLANGUAGE}}.svg
• {{FULLPAGENAMEE}}-{{REVISIONID}}.svg
• {{FULLPAGENAMEE}}-{{REVISIONTIMESTAMP}}.svg

or other combinations

Change 1011351 had a related patch set uploaded (by Gopavasanth; author: Gopavasanth):

[mediawiki/extensions/UrlShortener@master] Added appropriate filename on download of QR code

https://gerrit.wikimedia.org/r/1011351

I think the default filename should be {{PAGENAMEE}}.svg

That sounds like a good idea. When the QR Code is retrieved via the tools menu link on a page, then it'd be easy to use the page name and pass it to Special:QrCode. It could be a new filename parameter (alongside the existing url parameter) that'd take a filename and normalize it (including enforcing but not duplicating the .svg extension).

When generating a QR Code from the Special:QrCode form, however, it accepts any URL (conforming to $wgUrlShortenerAllowedDomains etc.) so there may or may not be an associated page title. That makes it harder to figure out a title for the download, but perhaps it could still inspect the URL for title=, and then fall back to qrcode.svg?

@Gopavasanth Looks like @Samwilson has left you some comments on the patchset. Have you had a chance to review the suggested changes?

Updates on the progress made during Wishathon (March 2024):

https://meta.wikimedia.org/wiki/Community_Tech/Wishathon/May_2023/Add_ability_to_share_QR_code_for_a_page_in_any_Wikimedia_project#March_2024