(designed to be a self Q and A, but of course, if you have a better solution other answers are appreciated).
I'd like a traditional (windows 98 era) "Start->All Programs" option in Windows 10. How do I re-create a facsimile of this in windows 10 without replacing my desktop manager entirely?
Right-click on an unused portion of the Windows Taskbar (by default at the bottom of your screen).
Select Toolbars->New toolbar...
Browse to either %ProgramData%\Microsoft\Windows\Start
Menu\Programs or %AppData%\Microsoft\Windows\Start Menu\Programs
(either option appears to combine both the "All Users" program menu
with the current User's program menu).
Click the Select Folder button
Unlock the taskbar if not already unlocked (via the right click menu of the taskbar).
Drag the new toolbar to the far left of the taskbar (windows, search and task view icons still appear to the left of it by default) and re-arrange your other toolbars (e.g. Quick Launch and the main taskbar).
Right-click on an empty portion of the new toolbar
Un-check both Show Text and Show title
Go to HKEY_CURRENT_USER -> Control Panel -> Desktop ->
WindowMetrics in the registry
Add a new String value (Reg_Sz) named MinWidth
Set to e.g. -360 (360 twips = 24 pixels)
Drag Programs toolbar size to minimum
Lock the taskbar
Log out and back in to windows
Reload Regedit.exe
Navigate to HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Streams and export the branch to a file
Unlock the taskbar, make the toolbar bigger, re-lock the taskbar
Log out and back in
Export HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Streams again as a new name
Use a diff tool (e.g. WinMerge or a github repo) to compare registry values. Mine differed in two spots of HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Streams\Desktop\TaskbarWinXP
The first difference location is probably where you wish to look at - in my case a hex pair value in the middle of my key changed from 40 to 9c. This is from 64 pixels to over 120 px.
Change that hex pair value to 18 (24 px) or something similar.
"Dirty" exit windows without allowing the registry to autosave. Accomplish this by control-alt-delete, select Task Manager, go to the details tab, then kill explorer.exe.
From Task Manager File->Run New Task and type c:\windows\system32\userinit.exe
Log off of windows or reboot to preserve your taskbar settings.
Delete or rename HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics\MinWidth or set to default value (-570 = 38px) (based on Is there any way to hide taskbar labels and not combine taskbar buttons?)
Log out and back in
I'm partial to Stardock, as I've had an ObjectDesktop ($30) subscription for years, so I'd recommend Stardock's Start10 ($5 if purchased separately).
Start10 gives users 2 choices for the classic start menu, the Windows 8 style, with transparency and other GUI affects, or the Windows 7 style.
Whichever option you chose, ensure you you manually add the start menu folders to the index, as Windows 10 does not have either location added by default.
