Mark 6:11 KJV
And whosoever shall not receive you, nor hear you, when ye depart thence, shake off the dust under your feet for a testimony against them. Verily I say unto you, It shall be more tolerable for Sodom and Gomorrha in the day of judgment, than for that city.
The New American Standard Bible, as well as the Revised Version omit the second part of the verse as published in the King James Version
Mark 6:11 NASB
Any place that does not receive you or listen to you, as you go out from there, shake the dust off the soles of your feet for a testimony against them.”
Mark 6:11 RV1885
And whatsoever place shall not receive you, and they hear you not, as ye go forth thence, shake off the dust that is under your feet for a testimony unto them.
Many (perhaps even most) modern versions emulate the Revised Version and simply omit the sentence in question, without any explanatory comment.
This is a complete sentence and yet it did not receive, in the Textus Receptus editions, a verse number of its own. It does not appear here in the majority of important codices, and Latin, Sahidic, and some Syriac and Boharic manuscripts.
It does, however, appear in some significant manuscripts, including two very old Latin manuscripts, and some Syriac and Boharic manuscripts, and with slight differences in minuscule 33 (9th century).
It was already doubted even before the KJV; this sentence does not appear in Wycliff (1380), the Bishops’ Bible (1568), and the Rheims (1582). Westcott and Hort omitted it and did not even mention it in their Appendix volume, nor is it mentioned in Scrivener’s Plain Introduction to Criticism of the New Testament, nor is it mentioned in Metzger’s Commentary, nor does it get even a footnote in the Souter or UBS Greek New Testament.
Henry Alford’s edition of the New Testament includes this sentence in the main text, but bracketed and italicized, with the brief footnote: “omitted in most ancient authorities: probably inserted here from Matthew 10:15.”
The same two sentences do appear, without any quibbling about their authenticity, in Matthew 10:14–15, and it is plausible that some very early copyist assimilated the sentence into Mark, perhaps as a sidenote subsequently copied into the main text.
In any case, its omission from Mark 6:11 does not effect its unchallenged presence in Matthew 10:15.