3.5 will be a Long Term Support (LTS) release; see release https://prometheus.io/docs/introduction/release-cycle/
There are a number of new features, and no known breaking changes in this release.

The following feature was removed since RC.1, since problems were discovered:

• [FEATURE] OTLP: Support promoting OTel scope name/version/schema URL/attributes as metric labels, via otlp.promote_scope_metadata. #16730 #16760

What's Changed

• [FEATURE] PromQL: Add experimental type and unit metadata labels, behind feature flag type-and-unit-labels. #16228 #16632 #16718 #16743
• [FEATURE] PromQL: Add ts_of_(min|max|last)_over_time, behind feature flag experimental-promql-functions. #16722 #16733
• [FEATURE] Scraping: Add global option always_scrape_classic_histograms to scrape a classic histogram even if it is also exposed as native. #16452
• [FEATURE] OTLP: New config options promote_all_resource_attributes and ignore_resource_attributes. #16426
• [FEATURE] Discovery: New service discovery for STACKIT Cloud. #16401
• [ENHANCEMENT] Hetzner SD: Add label_selector to filter servers. #16512
• [ENHANCEMENT] PromQL: support non-constant parameter in aggregations like quantile and topk. #16404
• [ENHANCEMENT] UI: Better total target count display when using keep_dropped_targets option. #16604
• [ENHANCEMENT] UI: Add simple filtering on the /rules page. #16605
• [ENHANCEMENT] UI: Display query stats in hover tooltip over table query tab. #16723
• [ENHANCEMENT] UI: Clear search field on /targets page. #16567
• [ENHANCEMENT] Rules: Check that rules parse without error earlier at startup. #16601
• [ENHANCEMENT] Promtool: Optional fuzzy float64 comparison in rules unittests. #16395
• [PERF] PromQL: Reuse histogramStatsIterator where possible. #16686
• [PERF] PromQL: Reuse storage for custom bucket values for native histograms. #16565
• [PERF] UI: Optimize memoization and search debouncing on /targets page. #16589
• [PERF] UI: Fix full-page re-rendering when opening status nav menu. #16590
• [PERF] Kubernetes SD: use service cache.Indexer to achieve better performance. #16365
• [PERF] TSDB: Optionally use Direct IO for chunks writing. #15365
• [PERF] TSDB: When fetching label values, stop work earlier if the limit is reached. #16158
• [PERF] Labels: Simpler/faster stringlabels encoding. #16069
• [PERF] Scraping: Reload scrape pools concurrently. #16595 #16783
• [BUGFIX] Top-level: Update GOGC before loading TSDB. #16491
• [BUGFIX] Config: Respect GOGC environment variable if no "runtime" block exists. #16558
• [BUGFIX] PromQL: Fix native histogram last_over_time. #16744
• [BUGFIX] PromQL: Fix reported parser position range in errors for aggregations wrapped in ParenExpr #16041 #16754
• [BUGFIX] PromQL: Don't emit a value from histogram_fraction or histogram_quantile if classic and native histograms are present at the same timestamp. #16552
• [BUGFIX] PromQL: Incorrect rounding of [1001ms] to [1s] and similar. #16478
• [BUGFIX] PromQL: Fix inconsistent / sometimes negative histogram_count and histogram_sum. #16682
• [BUGFIX] PromQL: Improve handling of NaNs in native histograms. #16724
• [BUGFIX] PromQL: Fix unary operator precedence in duration expressions. #16713
• [BUGFIX] PromQL: Improve consistency of avg aggregation and avg_over_time. #16569 #16773
• [BUGFIX] UI: Add query warnings and info to graph view. #16753 #16759
• [BUGFIX] API: Add HTTP Vary: Origin header to responses to avoid cache poisoning. #16008
• [BUGFIX] Discovery: Avoid deadlocks by taking locks in consistent order. #16587
• [BUGFIX] Remote-write: For Azure AD auth, allow empty client_id to suppport system assigned managed identity. #16421
• [BUGFIX] Scraping: Fix rare memory corruption bug. #16623
• [BUGFIX] Scraping: continue handling custom-bucket histograms after an exponential histogram is encountered. #16720
• [BUGFIX] OTLP: Default config not respected when otlp: block is unset. #16693