Composition of old cement
Old cement can have a variety of components. One is red lead, another is portland cement. Both create different problems, but both require that all removal of old cement should be done with breathing protection.
Removal of old cement can be attempted first with mechanical means. Stand the glass on edge and use a lead knife almost parallel to the surface of the glass to knock off the old cement. This works most of the time. But if the bond is too strong and begins to cause chipping of the glass, you need to switch to another method.
First consider whether the whole panel needs to be re-leaded. It may be that only portions require re-leading and so the remainder should be left in its original leads. This is especially true where there is painted glass, as you do not want to loose the original painting.
If mechanical removal is not working you need to consider chemicals.
Lye will dissolve linseed oil but perhaps also the paint and so should be used locally only and with extreme care and caution. Testing on unobtrusive areas is required.
Another way to soften the linseed oil is with heat and for this a steam generator works particularly well turning hard putty into the consistency of soft cheese which can be scraped off.
If it actually is Portland cement creating the bond then hydrochloric acid is what bricklayers use to dissolve and remove mortar stains from brickwork. Carefully paint it on the old cement and it should start fizzing and dissolving straight away.
What ever you decide to do, test first with some of the glass that will be discarded and confine your chemicals to the cement only and don’t spread them over the entire piece of glass. Wear a respirator at all times during the removal process.