Any real life examples these days of Tetralemma (Catuskoti) fourfold negation? What would be the fifth ('true') corner?
There is no corner, literally. It's an unsolvable tetralemma. There is no valid "solution" to it. Any "fifth thesis" designed to "solve" it is ruled out by the four theses themselves. A real example would be how, in real life, you can't have any sort of valid state or postulated entity that is not A, not not A, not both of those, and not neither of those. "All of the positions," or "all of the theses," or "all of the corners," are covered by and included in the "four theses" (四句).
The Buddha says that he does not praise even a tiny amount of existence, not even for as much as a finger's snap. Beings stuck on the polarity of "is" and "is not" actually entertain their own hidden theories of eternity and annihilation. These, referring to views concerning the existence of existence and nonexistence, are refined metaphysical versions of eternalism and annihilationism. They apply to "reality" instead of "my reality." When they are "my reality," they can be called "eternalist self-view" and "annihilationist self-view" respectively. In order to destroy attachment to the two views, the Buddha taught Venerable Kaccanagotta the teaching via the middle.
Over time, crafty men with clever, but not clever enough, minds started to come up with metaphysical theories that try to find the "quasi-existence" or "refined subtle existence" that was between "is" and "is not." They reformulated rejected imponderables of the Buddha into new philosophical refined forms. They said things like "The Tathagata both is and is not after death, that's why the Buddha declares that he neither is nor is not," or they said "The Tathagata has an ineffable vajra essence, neither existing nor not existing, that is forever unaltered both before and after the ending of the physical body." These are "both" and "neither," respectively, and it is part of the project of Ven Nagarjuna and his students to take these Buddhologies to task, to refute them, to show that these "new two" are merely the "old two" refurbished, and to correct these errors according to what they understand to be the truth of the Buddha's Dharma taught according to "the middle" -- from which "Madhyamaka" gets its name. The most likely opponent of the Madhyamaka polemics, the most likely locus for these outlandish theories, were the Pudgalavadins with their mysterious ineffable "pudgala," neither being nor not being, neither conditioned nor unconditioned. The Vimalāksa commentary to the MMK specifically frames Venerable Nagarjuna's interlocutor as being an Abhidharmika of the Saṃmitīya sect.