Confirmed. No coverage of the Avery 5126 (laser) 8126 (inkjet) 18126 (universal) all half sheet Letter size, in labels.xml

But I don't know it is really necessary to add any new manufacturers or fill in missing templates. But looks trivial if someone wants to. And from meta bug 120679 seems like there are other label Brands in need of curation.

A UX-advise question, but I'd say => WF

Otherwise, Avery offers document template for the size [1].

But trivial for an LO user to add their own user label [2][3]. Create one with the 8.5 x 5.5" size on Letter from Writer File -> New -> Labels dialog. 

No brand, and type is User. Then use the Format tab to enter dimensions. User can save that label format for reuse, the name assigned will be selectable from the "Brand" listbox on next use.

For a half-sheet Letter lable like the 5126/8126, this would be the stanza written to user profile, though I didn't include margins, might want the Avery template for exact:

<item oor:path="/org.openoffice.Office.Labels/Manufacturer"><node oor:name="my5126" oor:op="replace"><node oor:name="Label0" oor:op="replace"><prop oor:name="Name" oor:op="fuse"><value>[User]</value></prop><prop oor:name="Measure" oor:op="fuse"><value>C;21590;27940;21590;13970;0;0;1;1;21590;27940</value></prop></node></node></item>

If interested, the label elements are defined in source:
https://opengrok.libreoffice.org/xref/core/officecfg/registry/schema/org/openoffice/Office/Labels.xcs

And included labels are listed and defined:
https://opengrok.libreoffice.org/xref/core/extras/source/labels/labels.xml

=-ref-=
[1] https://www.avery.com/templates/5126
[2] https://help.libreoffice.org/25.8/en-US/text/shared/01/01010202.html

[3] is the legacy ASK thread in URL field

Created attachment 204996 [details]
Better description of pitch and margin to assign for a User label from the Writer Guide

Let's keep for volunteers. Seems to be a simple easy hack.

extras/source/labels/labels.xml with description in the header