Yes and no.

The classification of animal species (taxonomy) in the modern sense was pioneered by Carl von Linné (Linnaeus) in his work Systema Naturae (1735) which went through a number of revisions until reaching the famous 10th edition in 1758. Linnaeus sorted the animals into six major classes, which in turn were seperated into orders and genera. In the class Amphibia we find the order Serpentes (snakes), which contains six genera, one of which is Anguis. Into this generum Linné put both slow worms and worm snakes (which are properly snakes). You can find the slow worm (Anguis fragilis) on page 227 of the 1758 edition.

Why there? Let's have a look at how Linné defined the various orders and genera. My latin is abysmal, but there's a later English translation available. About the orders of Amphibiae we find:

I. REPTILES. These have feet, and flat naked ears without auricles.
II. SERPENTS. Feet 0; eggs connected in a chain, and penis double, muricate.

Then follows a long paragraph about snakes which contains biblical allusions as well. I believe I can make out another phrase in the original Latin edition which mentions something like "they are similar to lizards and frogs so that I can hardly differentiate".

So it is clear that at the time the science of anatomical taxonomy was not far enough developed. For Linné the fact that slow worms have no legs was the crucial element to decide they were a snake, not a lizard (or reptile — notice how the snakes are not a subset of the reptiles here). He did not think in terms of evolution, as is evident by the many comments about the divine creator (see also the introduction of the book).

Fast forward to the next giant of taxonomy, Georges Cuvier, and his work Le Règne Animal (published in 1817, 42 years before Darwin's work). In Volume II we read that the the third order of reptiles, the snakes are "are reptiles without feet" with an elongated body. Of these Anguis is the first order, of which he writes

Through their bony skull, teeth and tongue they resemble the lizards; they have triple eyelids, etc.; in a way they can be considered legless lizards.

The advanced anatomical knowledge of Cuvier enabled him to notice that slow snakes were actually lizards, not snakes. But the taxonomy had not caught up to this progress yet — Cuvier was strongly opposed to the early forms of evolutionary biology (like Lamarck's), so he would have rejected the idea that a species of lizards could lose their legs through an evolutionary process. Thus their order remained listed among the snakes for the time being.