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If the texture is a sparyed-on or roll-on application, there are strippers which will loosen and allow that to be scraped-off. If it is drywall mud, it's not coming off so easily and you will spend many hours and create a huge mess sanding that down with an orbital sander... then potentially have to re-skim to get the surface ready to accept a good tile job.
We do a lot of backsplashes in existing homes. When the substrate (the surface to which the tile is to be applied) is not satisfactorily flat and smooth, we find that the quickest, cleanest, simplest solution is to remove the drywall and replace with new, moisture-resistant drywall.
Looks like this may have been someone's remedy for a deteriorated / rotten mud sill (the flat wooden plate which sits atop the foundation wall, upon which the joists bear). If that is the case, it is possible that you might be able to remove the latter obstruction if one can restore the proper / original condition over the window. That would want to be examined and undertaken by a professional.
As with any such thing, it is difficult to say for sure without a full on-site examination by a home imporvement professional with structural knowledge and experience in these types of repairs.
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