This is not a RoProxy bug. Only certain Asset Types are supported with the inventory/list-json endpoint.
Asset types like Lua are usually not public in the first place. You can access yours (or a group you can manage) from this endpointwith authentication.
Also, why Lua? Those are scripts uploaded by Studio for source control, not created passes.
You need a Roblox Cookie to receive data from that endpoint, so it wont work through any proxies like RoProxy unless they allow you to place your .ROBLOSECURITY cookie from the proxy.
Ask yourself, are you sharing cookie-contents or authentication data. Are you sharing PII with the proxy. If not, it is safe. Rather than sending every URL you may use.
Responses that don’t include a content-type header should no longer error. However, the toolbox endpoints require authentication & I would recommend hosting your own instance of RoProxy Lite (don’t worry, that endpoint has pretty generous ratelimits.)
A few months ago RoProxy was automatically flagged by Bolster. Since RoProxy mirrors the Roblox site directly it was suspected of phishing - after reaching out to Bolster the flag has since been removed, but some antivirus software might still have it in their respective database. Regardless, any HTML pages now redirect to the Roblox site, so there’s no way you even could log in.
Currently, RoProxy doesn’t support scraping data from webpages. Any responses with an HTML content-type header will redirect to the official Roblox site to prevent users from entering sensitive information on RoProxy. There’s no need to use a proxy at all to check if a user has premium - look into the MembershipType property.