For reference:
[1] https://wiki.apache.org/tomcat/Specifications
[2] https://jcp.org/aboutJava/communityprocess/maintenance/jsr154/JSR154ChangeLog.html

There exists two versions of Servlet 2.5 specification [1]:
1) "Maintenance Release", 11 May, 2006
2) "Maintenance Release 2", 11 Sep, 2007

I do not have a copy of "1)", but my copy of "2)" (servlet-2_5-mrel2-spec.pdf) has "Version 2.5 MR6" on its front page. It aligns with change log numbering [2] and with "Change Log" section of the PDF file.

So I am looking at "Changes since Servlet 2.5 MR 5" section of the PDF.

There are two clarifications and one change to the deployment descriptor.

[quote]
The facet for http-method element in the deployment descriptor is currently more
restrictive than the http specification. The following change is being made to the
descriptor to allow the set of method names as defined by the http specification. The
pattern value of http-methodType is being changed from
<xsd:pattern value="[\p{L}-[\p{Cc}\p{Z}]]+"/>
to closely match what the http specification lists as allowable http methods names.
<xsd:pattern value="[&#33;-&#126;-[\(\)&#60;&#62;@,;:&#34;/
\[\]?=\{\}\\\p{Z}]]+"/>
[/quote]

This change is not present in our copy of  java\javax\servlet\resources\web-app_2_5.xsd  in Tomcat 6, nor in 7/8/trunk.

For later version of specification the correct value is present in web-common_3_0.xsd