Marlin 2.1 contains all patches from 2.0.9.4 in addition to…

New Features

• ✨ Support for up to 9 axes (#23112, #24036, #24231, #23975)

Improvements

• 🚸 Better M350, M114 with more axes (#23986)
• 🚸 Fix, improve, update ProUI (#24251)

Configuration

• 💥 Num Axes / Multi-Stepper based on Driver Types (#24106, #24120)
• 🔧 LEVEL_BED_CORNERS => LCD_BED_TRAMMING

Issues Fixed

• 🐛 Fix BACKLASH_COMPENSATION compile (#24072)
• 🩹 Fix some parameters w/out values (#24051)
• 🎨 Fix/adjust warnings (#24225)
• 🚩 MPC update (#24253)
• 💥 Change M306 M to M306 H (#24258)

Build / Upload

• 🔨 BTT STM32G0B1RE xfer build (#24245)
• 🩹 Fix EXTRUDERS 0 manual move compile (#24255)

For Developers

• 🏗️ Extend AXIS_CHAR to include E
• 🧑‍💻 MAP macro for axis lists, etc. (#24191)
• 🧑‍💻 Forward-compatible axis strings (167672d, #24259)
• 🧑‍💻 Combined axis strings