JIRA
JIRA is a proprietary issue tracking product that allows bug tracking and agile project management
An open source platform for developer collaboration.
Mattermost API is a free Documents & Productivity API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
REST API v4 — first-class per-endpoint pages generated from the canonical OpenAPI specification.
Details from api.mattermost.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
JIRA is a proprietary issue tracking product that allows bug tracking and agile project management
Web parser
Convert, merge, split, extract text and add page numbers for PDFs. Free for 250 documents/month
Programmatically access and update data inside a monday.com account
The Mattermost API is provided by Mattermost API v4 (api.mattermost.com). REST API v4 — first-class per-endpoint pages generated from the canonical OpenAPI specification.
Yes. The Mattermost API is listed in the Documents & Productivity category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Mattermost API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Mattermost API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Mattermost API is served over HTTPS, so it's safe to use in production.