Sometimes a and the take too much space in a slide, and I delete all of them to save me extra lines.
Is it a good practice?
There's nothing wrong with this: it's common practice in news headlines, and native speakers will find a well-worded sentence perfectly intelligible if the articles are removed. You must be careful about your choice of words, however, to avoid negatively affecting the clarity of your writing. It's probably not a problem as long as the text is relatively short, and presentation slides shouldn't have a lot of text, anyway.
Also, if you're going to remove any of the articles, you should probably remove all of them, otherwise it looks awkward, and might be interpreted as an error.
Yes, this is perfectly acceptable.
You can turn
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without any loss of meaning.
Yes, they may leave out articles for space. Note that in addition, even besides the sapce issue, it is sometimes more natural in English to leave out articles in titles (and slides often consist of what are effectively lists of titles).