In LO 22.214.171.124 (tested on W7) icons can be imported and assigned to toolbar items only under Toolbar icons size option Large but they only render as small. Under option Small, icons can't be imported. Tested with 24x24 and 26x26 png images. This mechansim has a long history of problems but they seem to begin after LO version 126.96.36.199. On UbuntuMate 18.04 with LO 188.8.131.52 and WinXp with LO 184.108.40.206, toobar icons are rendered properly but must be defined for each size. i.e. Tools > Options > View > Toolbar icon size; small then modify icon for each item in the toolbar; then select size large and repeat icon modifications. This sort of works with LO 220.127.116.11 on W10. If icons are assigned for both small and large then they do render completely but only at small size even on option large. If no icon has been assigned under option Small in LO 18.104.22.168 then one corner of the large icon is displayed. In all cases of improper rendering, immediately after the assignment the icon is rendered properly (at the current size option) but when LO is closed and reopened, this is replaced by the bad rendering.
Without doubt this problem begins after LO version 22.214.171.124. I replaced the LO 126.96.36.199 on the W7 computer with LO 188.8.131.52 and it acted exactly like this version does in Ubuntu-Mate. The problem begins after LO 184.108.40.206 and it is steadily getting worse with each release.
On the same W7 computer I installed OO 4.1.8 and found that it correctly renders small and large user-defined toolbar icons. Unlike LO, OO doesn't distinguish specific icon usage, i.e. for toolbars vs. others, but it does render correctly. The superior contextual awareness of LO is wasted if it screws up rendering.
I reproduce on Windows, but not on Linux.
Arch Linux 64-bit
Build ID: bd3aeaefff5e7bdef10c4702d1f388083557614e
CPU threads: 8; OS: Linux 5.9; UI render: default; VCL: kf5
Locale: fi-FI (fi_FI.UTF-8); UI: en-US
Built on 15 November 2020
Version: 220.127.116.11.alpha1+ (x64)
Build ID: 42a691933429dbb315de2bd7ba2724993c60411f
CPU threads: 2; OS: Windows 10.0 Build 19042; UI render: Skia/Raster; VCL: win
Locale: fi-FI (fi_FI); UI: en-US
On further investigation I see that all versions of LO in Windows after 18.104.22.168 screw up user-defined toolbar icons in one way or another by producing bad toolbar image files. I haven't tested any newer releases in Linux. Replacing these with ones produced by version 22.214.171.124 or hand-crafted (I use Gimp) fixes the problem. See https://ask.libreoffice.org/en/question/195561/toolbar-custom-icon-problem/
Ok, so let's call 6.1 the first bad release