On Tish’a B’Av, it is forbidden to wear sandals or shoes made from leather. According to many authorities, only leather shoes are forbidden, while those made from other materials, like rubber and plastic, are permissible, even if they are as good as leather shoes (Rif, Rosh).
Others hold that the prohibition applies to any comfortable shoe that keeps its wearer from feeling the roughness of the road (Razah). At the time of Chazal, only leather shoes were considered good. No one made good shoes from other materials; consequently, the prohibition did not apply to such shoes. Today, however, when [manufacturers] make shoes and sandals from other materials, and they are as good as their leather counterparts, one is forbidden to wear them on Tish’a B’Av. According to this opinion, one may not wear sneakers, “Source” (Shoresh) sandals, or the like on Tish’a B’Av, but one may walk around in slippers or canvas shoes with a thin sole, for one can feel the roughness of the road in them.
In practice, the accepted ruling is that non-leather footwear is permissible. Nonetheless, one who can, without difficulty, make do with slippers or canvas shoes that do not fully protect the feet should act stringently (M.B. 614:5).
Even according to the lenient opinion, one should not walk around in shoes or sandals made from synthetic leather, which look like [the real thing], because of mar’it ayin (doing something that appears to be forbidden).
A sick person or a postpartum woman, who are liable to catch a cold if they walk barefoot, may wear leather shoes. Similarly, one who [needs to] walk in a place where there is a possible danger of scorpions, or the like, may wear leather shoes. So too, one who [needs to] walk in a muddy place may wear regular shoes in order to avoid soiling his feet. A soldier on active duty may wear army boots (Sh.A. 554:17, 614:3-4). [The reason for all these leniencies is] because wearing shoes or sandals is prohibited [on Tish’a B’Av] only if one wears them for the sake of walking or comfort, but when there is another reason for wearing them, the prohibition does not apply. However, even when it is halachically permissible to wear shoes, one should not wear leather shoes if he can suffice with non-leather ones. After all, one may be lenient only when there is a need, and there is no need to wear leather shoes when one has shoes made from some other material.
Wooden shoes that are overlaid with leather are forbidden according to all opinions (Sh.A. 614:2). Many [poskim] hold that there is no prohibition [against wearing] sandals that have but one, thin strap of leather on top (K.H.C. 614:10).
For more on one who needs to wear leather shoes, see R. Eliyahu’s Hilchot Chagim 45:41-42; Minchat Shlomo vol. 1, 91:25:8; Hilchot Chag BeChag, Hilchot Yom HaKippurim 23:55, 58.
The author of Responsa Chelkat Ya’akov (2:83) permits one who wears orthotics lined with leather to insert them into canvas or rubber shoes, provided that the person suffers greatly when walking without orthotics. His reasoning: the orthotics are not part of the shoe, making the situation similar to one who stands on a leather cushion, which is permitted (Rama 614:2). Furthermore, since [failure to wear orthotics would cause] the person great pain, he can be compared to one who [needs to] walk through mud. See also R. Eliyahu’s Hilchot Chagim 45:41-42; Minchat Shlomo, vol. 1, 91:25:8; Hilchot Chag BeChag, Hilchot Yom HaKippurim 23:55, 58.