Earlier this year, Google announced that Universal Analytics (UA) will be going away and will stop processing new data after July 1, 2023.

Stack Exchange currently plans to switch to Google Analytics 4 (GA4). I won't re-iterate the problems with that plan; but why not take this opportunity to use a more suitable alternative, instead?

For price estimates, I'll be assuming 2400 000 000 monthly page views for all of Stack Exchange. Stack Overflow is responsible for most of that. This is way more than most companies list on their pricing page, so I've extrapolated; those prices are marked with "conjectured", and are probably overestimates.

Google Analytics 3 ("Universal Analytics")

• Closed-source JavaScript
• Hosted version ($12 500 per month) blocked by:
  • Firefox Tracking Protection
  • EasyPrivacy
• No self-hosted version
• Doesn't fill in the data gap
• Doesn't respect Do Not Track header

Google Analytics 4 ("GA4")

• Closed-source JavaScript
• Hosted version ($12 500 per month) blocked by:
  • Firefox Tracking Protection
  • EasyPrivacy
• No self-hosted version
• Attempts to fill in the data gap with magic machine learning, but can't do anything when the browser blocks the script.
• Doesn't respect Do Not Track header

Matomo

• Open-source JavaScript (BSD-3-Clause)
• Hosted version (conjectured €330 000 per month) blocked by:
  • EasyPrivacy (workaround exists)
• Self-hosted version (libre, GPLv3, PHP-based) blocked by:
  • None known.
• Can fill in the data gap from server logs.
• Respects Do Not Track header by default

GoAccess

• No JavaScript
• No hosted version
• Self-hosted version (libre, MIT; C, precompiled binary and Docker versions available) blocked by:
  • Nothing; cannot be blocked
• Has no data gap
• Can respect Do Not Track header, depending on server configuration
• No campaign / heatmap support

rrweb.io

• Open-source JavaScript
• No server-side software; you'll need to write your own.
• Blocked by:
  • None known.
• Measures All The Things; a useful tool, but definitely an informed consent opt-in thing, e.g. for reporting usability issues and site bugs. (If enabled for all users all of the time, it'd be a privacy violation, and your servers would buckle under the load.)

Fathom

• Closed-source JavaScript (minified, but with variable names preserved)
• Hosted version (conjectured $21 920 per month), blocked by:
  • EasyPrivacy (a workaround exists)
• Low data gap
• Does not respect Do Not Track header by default
• GDPR-compliant

Plausible

• Open-source JavaScript (MIT)
• Hosted version (conjectured $33 800 per month) blocked by:
  • EasyPrivacy (a workaround exists, but the EasyPrivacy maintainers don't like it)
• Self-hosted version (libre, AGPLv3, Docker-based) blocked by:
  • None known.
• Low data gap

Simple Analytics

• Open-source JavaScript
• Hosted version (conjectured £141 120) blocked by:
  • EasyPrivacy
• No self-hosted version
• Low data gap (with <noscript> fallback)
• Respects Do Not Track header by default

GoatCounter

• Open-source JavaScript (ISC)
• No known commercial self-hosted version, but you could ask.
• Self-hosted version (libre, modified EUPL, PostgreSQL/SQLite and Go-based) blocked by:
  • None known
• Near-zero data gap
• Ignores Do Not Track header

Piwik PRO (proprietary Matomo fork)

• Closed-source(?) JavaScript
• Hosted version (unknown price) blocked by:
  • EasyPrivacy
• On-premises version (unknown price) blocked by:
  • Unknown
• Unknown data gap (with <noscript> fallback)

Cloudflare Analytics

• It's Cloudflare. You don't use Cloudflare.
• It's only here for completeness; I have no idea how their pricing works.
• No data gap, by technicality.