Bug 59658

CacheIgnoreNoLastMod is supposed to ignore missing Last-Modified, but it only has any effect when there is also no ETAG on the response.

Manual:

Ordinarily, documents without a last-modified date are not cached. Under some circumstances the last-modified date is removed (during mod_include processing for example) or not provided at all. The CacheIgnoreNoLastMod directive provides a way to specify that documents without last-modified dates should be considered for caching, even without a last-modified date. If neither a last-modified date nor an expiry date are provided with the document then the value specified by the CacheDefaultExpire directive will be used to generate an expiration date.

caching.html:
If the response has a status of 200 (OK), the response must also include at least one of the "Etag", "Last-Modified" or the "Expires" headers, or the max-age or s-maxage directive of the "Cache-Control:" header, unless the CacheIgnoreNoLastMod directive has been used to require otherwise.



    else if (r->status == HTTP_OK && lastmods == NULL && etag == NULL && (exps
            == NULL) && (dconf->no_last_mod_ignore == 0) && !control.max_age
            && !control.s_maxage) {
        /* 200 OK response from HTTP/1.0 and up without Last-Modified,
         * Etag, Expires, Cache-Control:max-age, or Cache-Control:s-maxage
         * headers.
         */
        /* Note: mod-include clears last_modified/expires/etags - this
         * is why we have an optional function for a key-gen ;-)
         */
        reason = "No Last-Modified; Etag; Expires; Cache-Control:max-age or Cache-Control:s-maxage headers";
    }

Not really clear if this is a docs bug or not, but I lean towards doc.