SAP GUI 8.10 is released now and is supported until 30.06.2029.
The old SAP GUI 8.00 is supported until 31.07.2027.
General SAP GUI 8.10 help file reference; link. SAP blog page: link.
SAP GUI 8.10 changes
All formal changes are list in OSS note 3549180 – New and changed features in SAP GUI for Windows 8.10.
Custom theming
SAP GUI 8.10 allows custom theming. If you want to use it read the corresponding SAP help file.
32 vs 64 bit
Both are still supported, but only 1 per pc / laptop. See this blog.
Bex components
Bex components are removed from the default SAP GUI 8.10 build. If still needed, read OSS note 3691456 – SAP BW Business Explorer 7.x is no longer included with SAP GUI for Windows 8.10 DVD compilation and must be installed separately.