Is "walk on foot" acceptable?

I walked on foot for a long time is grammatically well-formed, but it's unlikely to occur for the reason Daniel δ has given.

"I walked on foot ..." is verbose; walked is superfluous in the presence of on/by foot.

However, the author may have used this for some reason in the context.

"Walk" implies "on foot." A more accepted usage is "go on foot." As opposed to "go by car."

On the other hand, redundancy can be used legitimately for emphasis. The specific case of walking for a long time appears to be one of the more typical uses of the phrase "to walk on foot." It's an admittedly informal corpus search, but in first 40 google hits (some overlapping) that I pulled up using the phrase "walked on foot," there were 12 separate usages where the phrase appeared to emphasize distance walked, and another six clearly contrasted walking with another form of transportation. There also seemed to be a group involving religious contexts.

Uses involving distance emphasis included:

• "He later walked on foot the length and breadth of Norway..."
• "They walked on foot for five days and nights from Gelati..."
• "...all 238 miles of them were walked on foot..."

Uses involving mode of transport contrast included:

• "...a mounted knight who rode on a horse and a foot soldier who walked on foot."
• "On this occasion Madame Zamenoy walked on foot, thinking that her carriage and horses might be too conspicuous..."
• "He walked on foot, rode on a donkey, or took a boat."

Uses involving religious context included:

• "...his Eminence the head of Islamic Supreme Council of Iraq walked on foot with visitors heading toward holy Karbala"
• "...walked on foot as far as the shrine..."

I think there are parallel cases with other redundant verb phrases that are in common use. The one I can think of offhand is "to call on the phone," but I'm sure there are others. Intuitively, I think our feelings about redundant phrases are on a continuum, with some phrases (e.g. "he ate with his mouth") seeming to require more impetus for the redundancy than others (e.g. "he walked on foot"). Given sufficient justification, such as if a person had previously been tube fed, there could be a reason to use the eating example, but it seems like the explicit contrast is more strongly required in that case.

On the contrary:

walking on foot makes sense as NOT being redundant when you think of walking on hand or walking on limb in fact some animals may even walk on tail

Yes, "he walked on foot" is good usage. It isn't redundant. "On foot" tells us how he walked, and what he was aware of.

He could "walk with his head in the clouds", or "walked through the meadow, his thoughts soaring with the birds". To my ear, "he walked on foot" shows a lesson who is aware of each step.

I might expect this to be emphasized later by foot pain or blisters, or more a detailed description of wet shoes opening to cold air as the stitching cuts through the soggy leather.

can we simply say "walk on foot" to mean go somewhere on foot?

This is a useful word:

OED:

tautology, n.

1. Unnecessary repetition, usually in close proximity, of the same word, phrase, idea, argument, etc. Now typically: the saying of the same thing twice in different words (e.g. ‘they arrived one after the other in succession’), generally considered to be a fault of style.

1686 J. Goad Astro-meteorologica i. xii. 56 The Taedium of Tautology is odious to every Pen and Ear.

Six Common Tautologies: https://blog.lingoda.com/en/six-common-tautologies/