Igrot Moshe, Responsa of Rabbi Moshe Feinstein Zt’l, Orech Chaim II 113
“Concerning that which you answered”

Now concerning that which you answered regarding the question raised in the book Omer HaShik’ha, regarding the fact that in both the Tur and Shulchan Orech there is no mention of a law stating that in our time, it is forbidden to spit on the Temple Mount:

(You offered as a possible answer) that in any event, such a case is only hypothetical because such a possibility could never occur since it is forbidden for tamei meitim (those rendered impure by exposure to death) to enter into the area of the Temple Mount, and (after all) we are all in that category today (and this would be the reason that the aforementioned codes of law did not mention this principle – namely, because it has no practical bearing) –

But observe, there is a place on the Temple Mount where entry for tamei meitim is permissible, even according to (the stricter) rabbinical ordinance – that is, up to the cheil, which lay within the soreg. Now, as far as that which the Rambam cites (Hilchot Beit HaBechira, Ch. 5,3) that the soreg encompassed all around, it is possible that the cheil was also encompassed all around. The Tosafot Yom Tov (Midot 2,3) explains it this way in the name of Rabbi Eliezer ben Yaakov --- according to this, it is permissible for tamei meitim to enter into the Mount until the soreg (which was a number of cubits wide – I have not been able to determine how many) from all sides, even from the west. And according to the opinion of the Rosh (ibid) who maintains that the soreg was only on the eastern side from north to south, perhaps the cheil was also only to be found there, and if that were the case, perhaps on the other sides the ten cubits of the cheil were past the Women’s Court – and thus, it would even be permitted (for the tamei met) to enter from the west.
Now regarding other forms of impurity (that may effect an individual), after all, it is possible to immerse (in a mikveh, and thus be purified of these other types of impurity, therefore the issue of other forms of impurity other than that of death is not an issue that could prevent one from entering into those areas of the Temple Mount which are permissible for tamei met to enter). If one is concerned (about the more severe state of impurity – not common in our times – ) about zivah, then that individual can count seven clean days, and immerses in a spring (or other natural source of fresh water) and afterwards wait until the following day (and that process would be sufficient even to allow a zav, a person with an unnatural flow, to enter into the Temple Mount as described above). And even though such an individual is still in the category of mechusar kapara, (a halachic category referring to one who has now been rendered pure but is still missing an element of atonement because he did not bring a required offering – that is not problematic, for) the Rambam cites in Hilchot Beit HaBechira 3,5 such an individual may still enter, even into the Women’s Court – (which possesses a higher level of sanctity) –and may certainly enter into the other areas of the Temple Mount.
Thus we find that the law which forbids spitting on the Temple Mount in the areas in which entry is permitted to tamei meitim, is indeed applicable…and thus, the question raised by the Omer HaShicha is a valid one.
(Rav Moshe now offers a possible explanation, in order to suggest an alternative answer to the question of the Omer HaShicha, as to perhaps why the issue is not addressed by the Tur and Shulchan Orech). Now perhaps it is somewhat unclear as to which wall (this is) – was there a clear tradition that it is the Western Wall of the Temple Mount, or was it perhaps the wall of the Azara behind the location of the Holy of Holies – and if that were so, indeed entry for tamei metim into that area would be forbidden, as you have postulated. Alternatively, it is also possible that this is the wall of one of the cells, an area that would be forbidden even according to the opinion of the Rosh.
I seem to recall that in the book She’alat Dovid by the Gaon, Rabbi Dovid of Karlin, the author expresses doubt as to which wall this is. Perhaps this is the reason why it is related that the Gaon of Brisk Z’tl did not visit the Kotel Ha Maarivi out of concern for impurity.
But I find this strange: in that place of our prayer, there is certainly a well established tradition that it is permitted to go there, dating from the time of our early sages – and how is it possible to differ with them?

With esteem,
Moshe Feinstein